Selective laser sintering (SLS) is a well-known process for rapidly producing three-dimensional objects from a variety of powder materials. Initially intended for fabricating prototype parts, SLS is now commonly used to fabricate end-use parts in a cost-effective and economical manner and with a high degree of accuracy. For example, SLS is used to fabricate ducting for environmental control systems such as for aircraft due to the lack of geometry constraints when forming such ducting. For example, ducting may have complex curvatures, shapes and/or intricate surface features that can readily be formed using SLS technology. SLS manufacturing is also used to fabricate relatively complex parts, both large and small, for a variety of industries including the automotive, aerospace, and medical industries. For example, fuel tanks can be readily formed using SLS as can dental prostheses, implants and crowns.
In SLS, an energy beam such as a laser beam is focused upon successively-applied layers of powder material. A wide range of powder materials can be used in SLS fabrication including, but not limited to, ceramic materials, metallic materials such as titanium, aluminum and alloy mixtures as well as polymeric materials such as polycrystalline polymeric powders. One such polymeric material commonly used in the production of end-use articles is nylon. In this regard, a relatively large portion of SLS parts are produced using commercial grades of nylon such as Nylon 11 and Nylon 12 due to their favorable mechanical properties (e.g., abrasion resistance) and chemical resistance.
During the SLS process, the laser beam is applied to a desired cross sectional region of the powder material using computer data such as a computer aided design (CAD) file of the finished build part. The application of the laser beam upon the powder material solidifies each layer and fuses the solidified layer to the previously solidified layer. During the SLS process, a vertically movable build platform which supports the powder material is incrementally lowered layer-by-layer as each layer is solidified. New layers of powder material are applied over previously solidified layers and are fused to the solidified layers. The process of incrementally solidifying layers is repeated until the build part is completed. The build part is contained with the unsolidified powder and is referred to as a part cake. The unsolidified powder is removed from the part cake to expose the finished build part following an appropriate cool down period.
Unfortunately, once the build part is completed, it is desirable to prevent the infusion of oxygen into the part cake which can cause oxidation of the build part. Oxidation may be undesirable the build part may be adversely affected and the build part may take on a brownish coloring. One method of preventing oxidation is to maintain the build frame in an inert atmosphere while the part cake cools. In this method, the build frame may be left inside the inert atmosphere of the SLS machine during cool down. Unfortunately, the cool down period can last up to several days during which time the SLS machine is unavailable for fabricating additional parts.
Various other methods have been employed to prevent oxidation of the build part but which allow for removal of the build frame from the SLS machine. One such method is to add anti-oxidant agents to the powder material. In the case of commercial grade nylon such as Nylon 12 powder, anti-oxidant agents are blended into the powder to prevent oxidation of the build part. Unfortunately, Nylon 12 powder is relatively expensive due to the addition of such anti-oxidants. The relatively high cost of Nylon 12 powder increases the end cost of parts as compared to other nylon grades such as Nylon 11 which lacks anti-oxidant agents but which exhibits improved mechanical properties.
Another drawback associated with the use of Nylon 12 is that certain SLS machine manufacturers employ a micro-chipping technique with the powder material. More specifically, some SLS machine manufacturers include a microchip in the powder canisters. The microchip is sensed by the SLS machines that are sold by the manufacturer such that the SLS machines cannot be operated unless the microchip is detected. In this manner, purchasers of the SLS machines must also typically purchase the micro-chipped powder at increased cost to the purchaser.
Other grades of nylon are commercially available but lack the microchip mechanisms. For example, Nylon 11 is similar to Nylon 12 but has several advantages over Nylon 12. For example, Nylon 11 is approximately one-third the cost of Nylon 12. Furthermore, Nylon 11 exhibits better mechanical properties as compared to Nylon 12. In one type of powder composition, Nylon 11 exhibits a tensile strength of 48 mega-Pascals (MPa) as compared to a tensile strength of 25 MPa for Nylon 12. Even further, Nylon 11 exhibits improved abrasion resistance, better impact strength and a lower tensile modulus as compared to Nylon 12.
Unfortunately, in order to make effective use of Nylon 11 in SLS manufacturing, methods must be employed to prevent oxidation of the build part. One solution includes the blending of anti-oxidant agents into the Nylon 11 powder similar to the agents that are added to Nylon 12. Unfortunately, the addition of anti-oxidant agents increases costs and production time due to the additional processing. A second solution for preventing oxidation of build parts fabricated from Nylon 11 includes waiting for an appropriate period of time for the build part to cool down in the inert atmosphere of the SLS machine. Unfortunately, as was earlier mentioned, doing so would prevent SLS fabrication during the 3-4 day cool down period resulting in an underutilization of the SLS machine.
As can be seen, there exists a need in the art for a system and method for enabling SLS part production using powder material that is devoid of anti-oxidant agents. More particularly, there exists a need in the art for a system and method of SLS part production using Nylon 11 which does not require the addition of anti-oxidant agents. Furthermore, there exists a need in the art for a system and method for fabricating build parts from Nylon 11 powder material without waiting for an extended period of time for cool down of the build part while the build frame is inside the SLS machine. Finally, there exists a need in the art for a system and method for fabricating build parts from Nylon 11 powder material which is simple, low in cost and which is convenient to use.